Eicosanoids are naturally occurring, optically active, bioactive metabolites which are produced by metabolism of arachidonic acid, the predominant 20-carbon fatty acid found in mammalian tissues. The investigation of prostaglandins (PG), thromboxanes (TX), lipoxins (LX) and leukotrienes (LT), collectively known as eicosanoids, has led to important advances in synthetic organic chemistry, biochemistry, and physiology.
Chemical methods for synthesizing and isolating optically active intermediates for use in preparing eicosanoids are known in the art. For example, Kobayashi et al. (1990) J. Org. Chem., Vol. 55, pages 5324-5335; Okamoto et al. (1988) J. Org. Chem., Vol. 53, pages 5590-5592; and Nicolau et al. (1985) J. Amer. Chem. Soc., Vol. 107, pages 7515-7518 describe the application of the Sharpless kinetic resolution, using titanium complexes, of racemic intermediates for synthesis of leukotriene B.sub.4, prostaglandins, and lipoxin A.sub.4, respectively. Similarly, Nori et al. (1990) Chemtracts, pages 173-197 describes the application of chiral reductive organometallic complexes for synthesizing prostaglandin chiral ancillary intermediates. In general, these methods suffer from a variety of deficiencies which include complicated multistep syntheses involving costly chiral reagents as well as complicated chemical resolution of racemic mixtures. More significantly, compounds having mediocre enantiomeric purities are produced.
Biocatalylic methods have been also applied towards the preparation and resolution of racemic eicosanoid intermediates. These methods exploit the regio- and stereo-selectivity of enzymes. The types of enzymes which are useful as biocatalysts and methods for their use in the preparation of optically pure compounds have been described in the literature. For a review, see, W. Boland et al. (1991) Synthesis, pages 1049-1072.
K. A. Babiak et al. (1990) J. Org. Chem., Vol. 55, pages 3377-3381 and EP application No. 0357009, for example, describe the resolution of racemic prostaglandin synthons, e.g. 4-hydroxy-2-alkyl-2-cyclopentenone, using lipase-catalyzed esterification reactions in the presence of vinyl acetate as organic solvent and acylating agent. Selective acylation of only one enantiomer occurs during the reaction, allowing isolation of optical isomers having enantiomeric excesses greater than 99%.
In practicing this resolution method, however, the crude reaction mixture, containing a mixture of diastereomers, must be immediately purified upon completion of the esterification reaction to prevent epimerization of the acylated derivative. Conventional purification methods such as column chromatography over silica gel is generally used for separating the diastereomers. While such purification methods are generally useful in small scale reactions, they are impractical for industrial scale operations.
Processes for isolating organic compounds from crude mixtures are known in the art. Among these methods, processes which involve preferential complexation of organic compounds with metal salts have been described. For example, U.S. Pat. No. 4,452,994 discloses the use of lithium salts for purifying prostaglandin racemates from crude reaction mixtures; U.S. Pat. No. 4,057,541 relates to the use of calcium salts for separating 3-hydroxy and 3-keto steroids from reaction mixtures; and Sharpless et al. (1975) J. Org. Chem., Vol. 40, pages 1252-1257 relates to the use of calcium and manganese salts for preferential complexation with alcohols. These metal complexes have been used in processes for purification of racemates from complex reaction mixtures. However, prior to the present disclosure, there are no teachings or suggestions concerning the use of metal salt complexation as a means of separating enzymatically or chemically resolved optical isomers from crude reaction mixtures.
Accordingly, there is a need in the art for a practical method for recovering optical isomers from enzymatically resolved reaction mixtures which avoids at least one of the aforementioned deficiencies.